The role of calreticulin (CALR) in vertebrate hematopoiesis

Abstract

Introduction: Calreticulin (CALR) is a multi-functional protein mainly controlling protein folding and calcium homeostasis in endoplasmic reticulum. Recently, recurrent mutations in CALR have been found in essential thrombocythemia (ET) and primary myelofibrosis (PMF) patients without Janus kinase 2 (JAK2) or thrombopoietin (MPL) mutations. Previous studies have shown that CALR mutation might act through the JAK2-STAT5 signaling pathway in myeloproliferative neoplasm. However, the functional role of CALR in hematopoiesis remains unknown. In this study, we made use of zebrafish model to examine the unknown roles of calr in vertebrate hematopoiesis. Methods: Gene expression of calr and other hematopoietic genes were evaluated by whole-mount in situ hybridization (WISH) and quantitative RT-PCR (Q-PCR). Morpholino (MO) and transcription activator like effector nuclease (TALEN) were used to knock down and knock out calr. Results: Zebrafish calr is expressed predominantly in head region during embryonic stage. It is also expressed in the hatching glands and axial vasculature at 18-24 hours post-fertilization (hpf), in caudal hematopoietic tissue (CHT) at 48hpf and in the lateral line in later stage. In adult zebrafish, calr is highly expressed in kidney, liver, intestine, muscles and skin tissues. Knock-down of calr resulted in a decrease in expression of genes associated with myeloid lineages at 24hpf, including l-plastin, mpo and mpeg1. It also led to an increase in expression of c-myb (associated with HSCs) at 48, 72 and 96hpf. To confirm these MO-mediated phenotypes and further examine the role of calr in adult hematopoiesis, TALENs targeting exon-1, -5 and -9 were used to generate calr knock-out models and their mutagenic activities were shown by restriction fragment length polymorphism (RFLP) assay. Conclusion: Knock-down of calr altered myeloid and HSCs lineages during zebrafish embryonic development. It provides important ground for further understanding of the unknown role of Calr in vertebrate hematopoiesis